Liquid junction ion electrodes are used to detect the activity or concentration of a specific ion contained in a test solution. A typical pH meter will use a pair of electrodes. One of the electrodes is a reference electrode, and the other electrode is an ion-sensitive electrode that chemically reacts with a specific ion in the test solution. When the electrodes are immersed in the test solution, the resulting chemical reaction creates a measurable electrical potential across the two electrodes that is displayed on the meter.
Typical liquid junction reference electrodes consist of a closed glass tube having a porous glass plug at one end. An electrical conductor passes through the glass tube to make electrical contact with a reference solution that fills the interior of the glass tube. Although these conventional glass reference electrodes have functioned well, their large size and expense make them incompatible for applications utilizing the recently developed ion-sensitive field effect transistor, known as an ISFET. The unavailability of suitable miniature ion electrodes has restricted the appilcation of the ISFET as a viable chemical sensor in the industrial and biomedical fields.
Recent attempts have been made to manufacture miniaturized liquid junction reference electrodes using semiconductor fabrication techniques. One method is described by Smith et al. in "A Solid State Miniature Reference Electrode," IEEE NSF Symposium on Biosensors -- 1984, pages 61-62 (1984), and in "Integrated Sensor for Electromechnical Measurements," IEEE Transactions on Biomedical Engineering, Vol. BME 33, No. 2, pages 83-89, Feb. 1986, wherein a cavity housing a solution in a silicon base is covered with a porous silicon membrane. The major drawback to this configuration, as indicated in the above-referenced publications, is the lack of control over the pore size and density of the silicon membrane, resulting in erratic electrode performance. Another difficulty encountered in the manufacture of these silicon-based electrodes is achieving a leak-proof bond between the membrane and the silicon body.
Attempts have also been made to develop a solid state reference electrode using parylene coating on the gate of the ISFET, or using a thin-film silver/silver-chloride electrode that is either in direct contact with a test solution of a chloride ion or is covered with a polymer saturated with a chloride ion. In practice, these electrodes have failed to perform as well as the conventional glass liquid junction electrodes because of limited useful ranges in the primary ion concentrations and instability in the electrode potential due to changing concentration of the primary ion. The present invention overcomes these and other disadvantages in the manufacture and application of miniaturized silicon-based liquid junction reference electrodes. The present invention can also be employed in the manufacture of liquid junction ion-sensitive electrodes.